Measurement of Accumulation of Antibiotics toStaphylococcus aureusin Phagosomes of Live Macrophages

Staphylococcus aureus ( S. aureus ) has evolved the ability to persist after uptake into host immune cells. This intracellular niche enables S. aureus to potentially escape host immune responses and survive the lethal actions of antibiotics. While the elevated tolerance of S. aureus to small-molecule antibiotics is likely to be multifactorial, we pose that there may be contributions related to permeation of antibiotics into phagocytic vacuoles, which would require translocation across two mammalian bilayers. To empirically test this, we adapted our recently developed permeability assay to determine the accumulation of FDA-approved antibiotics into phagocytic vacuoles of live macrophages. Bioorthogonal reactive handles were metabolically anchored within the surface of S. aureus, and complementary tags were chemically added to antibiotics. Following phagocytosis of tagged S. aureus cells, we were able to specifically analyze the arrival of antibiotics within the phagosomes of infected macrophages. Our findings enabled the determination of permeability differences between extra- and intracellular S. aureus , thus providing a roadmap to dissect the contribution of antibiotic permeability to intracellular pathogens.